THE PLEURA. ,79 



By some other consentaneous influence, the spinal accessory nerve likewise exerts 

 its power, and the sterno-maxillaris muscle is stimulated by the anterior division of it, 

 and the motion of the head and neck corresponds with and assists that of the chest; 

 while the posterior division of the accessory nerve, by its anastamoses with the motor 

 nerves of the levator humeri and the splenius, and many other of the muscles of the 

 neck and the shoulder, and by its direct influence on the rhomboideus, associates 

 almost every muscle of the neck, the shoulder, and the chest, in the expansion of the 

 thorax. These latter are muscles, which, in undisturbed respiration, the animal 

 scarcely needs ; but which are necessary to him when the respiration is much 

 disturbed, and to obtain the aid of which he will, under pneumonia, obstinately stand 

 until he falls exhausted or to die. 



The cavity of the chest is now enlarged. But this is a closed cavity, and between 

 its contents and the parietes of the chest a vacuum would be formed ; or rather an 

 inequality of atmospheric pressure is produced from the moment the chest begins to 

 dilate. As the diaphragm recedes, there is nothing to counterbalance the pressure of 

 the atmospheric air communicating with the lungs through the medium of the nose 

 and mouth, and it is forced into the respiratory tubes already described, and the lungs 

 are expanded and still kept in contact with the receding walls of the chest. There is 

 no sucking, no inhalent power in the act of inspiration; it is the simple enlargement 

 of the chest from the entrance and pressure of the air. 



From some cause, as inexplicable as that which produced the expansion of the 

 chest, the respiratory nerves cease to act; and the diaphragm, by the inherent 

 elasticity of its tendinous expansion and muscular fibres, returns to its natural form, 

 once more projecting its convexity into the thorax. The abdominal muscles, also, 

 which had been put on the stretch by the forcing of the viscera into the posterior part 

 of the abdomen by means of the straightening of the diaphragm, contract, and 

 accelerate the return of that rnuscle to its quiescent figure ; and the ribs, all armed 

 with elastic cartilages, regain their former situation and figure. The muscles of the 

 shoulder and the chest relax, a portion of the lungs are pressed on every side, and the 

 air with which they were distended is again forced out. There is only one set of 

 muscles actively employed in expiration, namely, the abdominal : the elasticity of 

 the parts displaced in inspiration being almost sufficient to accomplish the purpose. 



The lungs, however, are not altogether passive. The bronchial tubes, so far as 

 they can be traced, are lined with cartilage, divided and subdivided for the purpose 

 of folding up when the lungs are compressed, but elastic enough to afford a yieldinof 

 resistance against both unusual expansion and contraction. In their usual state the 

 air-tubes are distended beyond their natural calibre; for if the parietes of the thorax 

 are perforated, and the pressure of the atmosphere rendered equal within and without 

 them, the lungs immediately collapse. 



THE PLEURA. 



The walls of the chest are lined, and the lungs are covered by a smooth glistening 

 membrane, the pleura. It is a .serous membrane, so called from the nature of its 

 exhalation, in distinction from the mticnus secretion yielded by the membrane of the 

 air-passages. The serous membrane generally invests the most important organs, 

 and always those that are essentially connected with life ; while the mucous mem- 

 brane lines the interior of the greater part of them. The pleura is the investing 

 membrane of the lungs, and a mucous membrane the lining one of the bronchial tubes. 



Among the circumstances principally to be noticed, with regard to the pleura, is 

 the polish of its external surface. The glistening appearance of the lungs, and of the 

 inside of the chest, is to be attributed to the membrane by which they are covered, 

 and by means of which the motion of the various organs is freer and less dangerous. 

 Although the lungs, and the bony walls which contain them, are in constant approxi- 

 mation with each other, both in expiration and inspiration, yet in the frequently 

 hurried and violent motion of the animal, and, in fact, in ever}' act of expiration and 

 inspiration, of dilatation and contraction, much and injurious friction would ensue if 

 the surfaces did not glide freely over each other by means of the peculiar polish of 

 this membrane. 



Every serous membrane has innumerable exhalent vessels upon its surface, from 

 ■which a considerable quantity of fluid is poured out. In life and during health it 

 exists in the chest only as a kind of dew, just sufficient to lubricate the surfaces. 



